1. Field of the Invention
The present invention relates to a scroll-type fluid displacement apparatus, and more particularly, to an anti-wear plate mechanism for the spiral elements of the scroll members used in a scroll-type fluid compressor.
2. Description of Related Art
Scroll-type fluid displacement apparatuses are known in the art. For example, U.S. Pat. No. 5,249,943, which disclosure is incorporated herein by reference, discloses a basic construction of a scroll-type fluid displacement apparatus including two scroll members, each having an end plate and a spiroidal or involute spiral wrap element extending from the end plates. The scroll members are maintained angularly and radially offset so that both spiral elements interfit to form a plurality of line contacts between their spiral curved surfaces to thereby seal off and define at least one pair of fluid pockets. The relative orbital motion of the two scroll members shifts the line contact along the spiral curved surfaces and, as a result, changes the volume in the fluid pockets. The volume of the fluid pockets increases or decreases depending on the direction of orbital motion. Thus, the scroll-type apparatus is able to compress, expand or pump fluids.
In comparison with conventional piston type compressors, scroll-type compressors have certain advantages. For instance, they use fewer parts and allow continuous compression of fluid. However, one of the problems with scroll-type compressors is difficulty in sealing the fluid pockets. Axial and radial sealing of the fluid pockets must be maintained in a scroll-type compressor in order to achieve efficient operation. The fluid pockets are defined by line contacts between the interfitting spiral elements and axial contact between the axial end surface of one spiral element and the inner end surface of the facing end plate.
Various techniques have been used to address the sealing problem, in particular, that relating to axial sealing. In U.S. Pat. No. 3,994,636, which disclosure is incorporated herein by reference, a seal element is mounted in a groove formed in the axial end surface of each spiral element. An axial force urging means in each groove, such as a spring, urges the seal element towards the facing end surface of the end plate, thereby effecting an axial sealing.
Because the seal element disclosed in the above patent is urged towards the facing end surface of the end plate by a spring or other axial force urging mechanism, over period of time, wear occurs between the end surface of the seal element and the end plate of the scroll member, especially when a lightweight alloy, such as an aluminum alloy, is used as the material for the scroll member.
One solution to these problems with respect to wear is disclosed in U.S. Pat. No. 4,047,855, which disclosure is incorporated herein by reference. This patent discloses an involute anti-wear plate disposed between the axial end surface of the spiral element and the inner end surface of the opposite end plate. The involute anti-wear plate covers the area of the surface of the end plate where the other spiral element makes axial contact during orbital motion. Excessive wear or abrasion of the end plate is thereby prevented.
It should be noted that, in scroll-type fluid compressors, the interfitting spiral elements, normally constructed of lightweight alloys, such as an aluminum alloy, are subject to several temperature zones which are caused by the increasing pressure and decreasing volume as fluid moves to the center of the compressor. The greatest temperature exists in the center of the compressor, as this pocket has the smallest volume and largest pressure. This causes greater thermal expansion at the center of the spiral elements than at any other portion. Because the thermal expansion coefficient of aluminum alloy is generally greater than that of steel, aluminum will be affected more by temperature changes than steel. As the center of the spiral element expands thermally, the center of the involute anti-wear plate is subjected to higher stress than the outer radial portions. As a result, the center of the spiral element is more easily subjected to damage and failure.
Further, Japanese Utility Model Patent No. JP SHO63-41589, which disclosure is incorporated herein by reference, discloses a scroll-type compressor in which an anti-wear plate is also disposed between the axial end surface of the spiral elements and the inner end surface of the opposite end plate. The axial end surface of the spiral element includes a recessed portion formed at the center thereof. The recessed portion is such that the anti-wear plate is more deeply placed according to the thermal expansion of the spiral elements. Thereby, the construction could absorb the thermal expansion by the partial depression of the anti-wear plate.
However, this construction allows the center of the involute anti-wear plate to be repeatedly subjected to high stress due to bending. Thus, the center portion of the anti-wear plate is easily subject to abrasion, damage, and failure.